The invention relates to a device for temperature measurement.
Such devices which are known in the art for contactless temperature measurement comprise a detector for receiving heat radiation emanating from a measurement spot on an object of measurement, an optical system for imaging the heat radiation emanating from the measurement spot onto the detector and a sighting arrangement for identifying the position and size of the measurement spot on the object of measurement by means of visible light. A further processing arrangement which converts the detector signal into a temperature indication is also connected to the detector.
In this case the optical system is so designed that at a certain measurement distance for the most part only heat radiation from a certain area of the object of measurement, namely the so-called measurement spot, is focussed onto the detector. In most cases the size of the measurement spot is defined by the area from which 90% of the heat rays focussed onto the detector strike. However, applications are also known in which there are reference to values between 50% and 100%.
The pattern of the dependence of the size of the measurement spot upon the measurement distance depends upon the design of the optical system. A fundamental distinction is made between distant focussing and close focussing. In distant focussing the optical system images the detector into infinity and in close focussing it images it onto the focus plane. In the case of distant focussing it is necessary to deal with a measurement spot which grows linearly with the measurement distance, whereas in close focussing the measurement spot will first of all become smaller with the measurement distance and after the focus plane will enlarge again if the free aperture of the optical system is greater than the measurement spot in the focus plane. If the measurement spot in the focus plane is greater than the free aperture of the optical system, then the measurement spot is also enlarged with the measurement distance even before the focus plane. Only the increase in the size of the measurement spot is smaller before the focus plane than after it.
In the past various attempts were made to render the position and size of the measurement spot, which is invisible per se, visible by illumination. According to AJP-A57-22521 a plurality of rays which originate from several light sources or are obtained by reflection from a light source are directed along the marginal rays of a close-focussed optical system onto object of measurement. In this way the size and position of the measurement spot for a close-focussed system can be rendered visible by an annular arrangement of illuminated points around the measurement spot. U.S. Pat. No. 5,368,392 describes various methods of outlining measurement spots by laser beams. These include the mechanical deflection of one or several laser beams as well as the splitting of a laser beam by a beam divider or a fibre optic system into several single beams which surround the measurement spot.
A sighting system is also known in the art which uses two laser beams to describe the size of the measurement spot. This system uses two divergent beams emanating from the edge of the optical system to characterise a close-focussed system and two laser beams which intersect in the focus point to characterise a close-focussed optical system.
All known sighting arrangements are either only useful for a certain measurement distance or require relatively complex adjustment and are often quite expensive.
The object of the invention, therefore, is to make further developments to the device for temperature measurement in such a way as to facilitate simple identification of the position and size of the measurement spot independently of the distance.
This object is achieved according to one aspect of the invention, in that the sighting arrangement has a diffractive optical system for producing a light intensity distribution with which the position and size of the measurement spot on the object of measurement can be rendered visible.
According to another aspect of the invention, a diffractive optical system is an optical element, the function of which is based principally upon the diffraction of light waves. In order to produce the diffraction, transverse microstructures which can consist, for example, of a surface profile or a refractive index profile are provided in the optical element. Diffractive optical elements with a surface profile are also known as so-called holographic elements. The surface patterns are produced for example by exposure of photoresist layers to light and subsequent etching. Such a surface profile can also be converted by electroplating into an embossing printing block with which the hologram profile can be transferred into heated plastic films and reproduced. Thus many holographic elements can be produced economically from one hologram printing block.
The pattern of the diffractive optical system is produced by interference of an object wave with a reference wave. If for example a spherical wave is used as the object wave and a plane wave as the reference wave then an intensity distribution is produced in the image plane which is composed of a point in the centre (Oth other), a first intensive circle (first order) and further less intensive circle of greater diameter (higher orders). By screening out of the Oth and the higher orders an individual circle can be filtered out. A plurality of other intensity distributions which are explained in greater detail below with reference to several embodiments can be produced by other object waves.
According to another aspect of the invention, usually approximately 80% of the energy emanating from the light source lies in the patterns produced by the diffractive optical system. The remaining energy is distributed inside and outside the measurement spot.
According to a further aspect of the invention, the light intensity distribution which is produced can be formed, for example, by a circular marking surrounding the measurement spot or a cross-shaped marking.
Such a device can also be produced economically and only requires a little adjustment work.
Further constructions of the invention are the subject of the subordinate claims and are explained in greater detail below with reference to the description of several embodiments and to the drawings.